Basement media room - initial plan. Thoughts?

Going 7.1 is what I am recommending for all new systems; I'd reverse it and say why not? 7.1 content will keep increasing, we're not talking about a really forward-looking system with front wides and front heights. 7.1 is a solid way to setup and plan a theater space these days. For budget reasons, you can always properly run and wire for the 7.1 system and simply add the 'extra' two speakers in the future. The only thing that might be an issue is that for the 7.1 system you have those two single seats right on the side walls, and they won't be in a great position for the side surrounds.

The open back plan is just fine, tons of people have open theater concepts. You did correctly notice that your sub might not be able to keep up, so just know that. Similar to the 7.1 add some extra wiring to a few additional locations so you can add and/or test various subwoofer locations. Also, with an open plan, things like sound isolation are not going to be part of the process (unless you intend to sound isolate the entire basement).

Screen size is very personal. If you are trying to follow exact standards, then you need to consider viewing angles, distances, etc... If you are not as concerned with that precision, then it is how big you want in the room you have. Also, I highly doubt that the size screen you decide to use will have any ability to sacrifice your IQ.

I have 136" wide painted-wall screen on similar 'open' basement theater. Sit approx. 12-13' back and NO WORRIES about screen too big, dim or low resolution. DVD looks good, Blu-ray is superb. I have Optoma 1080p projector as well. Personally, I believe the most important factor is simply light control. I have a long room that is multi-tasked behind seating like yours, and can have can lights on above or behind seating for gameday scenarios and the screen is plenty bright for viewing. Keep colors (especially around screen) dark, go with black screen wall if possible and if you can manually mask the screen that helps as well and you shouldn't desire to do something different after all is done.

Another beauty of using a painted screen wall technique is the ability to try out a size to your liking first and it's simply a matter of repainting if it's too small or large. I will NEVER go smaller than 10 ft wide again.

Here's a crappy iphone pic of multi-purpose basement room with large sectional. We move the chaise lounge section for kids party movie nights. Otherwise, it's about the biggest screen and most versatile room I could think of having. The colors work well also.. chocolate berber carpet, gold walls, chocolate ceiling, and black screen wall.

How tall is the ceiling in that picture? (the one with the sectional couch)?

About 8 foot.

I was off on screen.. it's 136" diag. My bad, been a while since the build. Still pretty big, especially built 8 years ago. Starting idea / design phase of next house and anticipating 4k projectors within 2-3 years becoming available. Anyone do at-home IMAX yet????

Don't worry about the power ratings, you are fine there. Couple things:
- The KEF Q900's are listed as 8ohm speakers but are really 4ohm and your receiver should be purcahsed with 4ohm in mind:
http://www.stereophile.com/content/kef-q900-loudspeaker-measurements

- The Onkyo TX-NR717 seems like a nice receiver, but it is lacking Audyssey MultiEQ. It just has 2EQ. I would open my options up to other brands like Denon as well that have Audyssey MultiEQ and MultiEQ XT within your price range. For example:
http://www.accessories4less.com/make-a-store/item/DENAVR3312CI/DENON-AVR-3312CI-7.2-Integrated-Network-A/V-Surround-Receiver/1.html

Authorized dealer who sells refurbished models with full factory warranty. That is a lot of receiver for $600

What are you thinking for a subwoofer?
I like the KEF speakers, but have you considered other options such as JTR, or Chase Home Theater?

You also need to consider the size of your screen, the ceiling height, and the size of your center channel. Have you considered building a simple false wall and hiding the speakers behind an accoustically transparent screen? This opens up a few options. For one, it keeps the speakers out of the way of the kids, and more important, it allows you to position the speakers at the proper height. And it allows you to buy a matching third floor-standing Q900 and bypass the horizontal center. A vertical matching speaker is preferred. You almost always get better imaging and off-axis response. Some things to read:
http://www.audioholics.com/education/loudspeaker-basics/vertical-vs-horizontal-speaker-designs
http://www.avsforum.com/t/837848/minimalist-approach-to-screen-wall

You can do an accoustically transparent screen either via DIY for about $350-400 easily, or order one through a company like Jamestown for about $500-600 shipped depending on screen size. The SeymourAV Centerstage XD screen material is excellent.

I would also read into the Acer HD9500 projector. It will offer better blacks vs the Optoma and it has some lens shift as well making it easier to place without using keystone. In my room (8ft ceilings, 125" 16:9 screen) there was no way to place the Optoma without using keystone. Since this review, Acer has added a better color management system and improved sharpness:
http://www.projectorreviews.com/acer/h9500bd/

I know Art gave it the top PJ award in it's price range and you can find it around $1500 shipped typically. Just make sure it has the latest firmware.

I also agree with the others and go ahead and wire for 7.1 but spend the bulk of your speaker budget on your front main speakers and your subwoofer. Surrounds should be the least amount of your speaker budget.

Sometimes you really have to dig for the info. For Onkyo they are typically very forthcoming on their features/specs pages on their website:
http://www.us.onkyo.com/model.cfm?m=TX-NR717&class=Receiver&p=f
About 5 lines down " Certified 4ohms performance ". Now, keep in mind if you buy 8ohm rated speakers this won't be as big a concern so it really depends on what speakers you buy. My advice is to decide on speakers first, and that will help dictate what receiver you need. Onkyo, Yamaha, Denon, Pioneer, are all respected receiver brands and offer similar features and performance. I do recommend the Audyssey (or other room correcting software) feature and look for MultiEQ at a minimum. You can read more about Audyssey, but from my experience, when properly implemented, it can make a difference in terms of audio quality. It is also very easy to use.

The dedicated speaker and subforum will assist

I highly recommend the false wall approach with an accoustically transparent screen, especially with kids and keeping the wife happy. The added benefit, as I mentioned, is using a vertical matching speaker and getting perfect placement. So many times people buy horizontal center speakers and then place it too high, or too low, and then are challenged trying to hear dialogue or getting good off-axis response (meaning if you move to the sides of the speaker and are not sitting directly in front, you get poor sound). This was something that challenged me in my last media room, and I can honestly say the vertical center channel with the tweeter placed behind the screen at ear level has made a big difference. Another option to is to use an in-wall speaker behind an AT screen.

As for Jamestown, you can call him and discuss screen size and AT screen material or buy the AT screen and frame or buy from his Ebay store:
http://www.ebay.com/itm/Projector-Screen-120-Acoustically-transparent-Fixed-HD-/330792721297?pt=US_Projection_Screens_Material&hash=item4d04c85b91

Attached is some links to my screen and speakers behind my Jamestown screen. My wife preferred in-wall vs false wall, so I went that route but false wall does give you more speaker flexibility:
http://www.avsforum.com/t/1341864/acks-media-basement-build/150#post_21514843
http://www.avsforum.com/t/1341864/acks-media-basement-build/150#post_21539532

It depends on the specific projector and individual. Some projectors are move forgiving with keystoning, and some are not. I have owned both As a general rule, I try to avoid keystone where possible, and lens shift is a really nice feature when setting up a projector if you can get it. I have found this calculator to be very helpful and it has the Optoma projector you are considering in it:
http://www.eliteprojectorcalculator.com/